The invention relates to offshore structures and more particularly to a support structure for supporting wells, production facilities, and/or drilling rigs located in a body of water.
In the drilling and production of hydrocarbons and other fluid minerals at offshore sites, offshore platforms are installed for the support of the necessary above water facilities and related equipment to accomplish such drilling and production. Often the size, weight and number of facilities and equipment preclude the use of minimal support structures having less than three legs anywhere between the sea floor and the facilities deck. Support structures having less than three legs must resist eccentric gravity loads and lateral environmental loads such as wind, wave, current and seismic by developing internal bending moment in the legs, whereas the support structures having three or more legs resist the loads primarily be developing internal axial loads. Support structures having three or more legs are stiffer and more efficient than the minimal structures for any loading other than very minimal loads. Production facilities are especially sensitive to motion which occurs under cyclic or intermittent loading and are adversely affected by the motions of the more flexible minimal structures. As is well-known in the art, an active wave zone exists as part of the surface of a body of water. Such wave zone produces loading on production and drilling facilities and other support structures located in a body of water.
Given the need for support structures which have three or more legs, current technology has provided conventional pile-supported jackets. Jackets having three legs are called tripods. Decks can be connected to the jacket by conventional methods, including transitions which allow decks to have a different number of legs than the jacket has.
Tripods have the advantage of presenting less surface area to wind, wave and current than a structure having more legs. Tripods are, however, inefficient in resisting the applied lateral loads and applied overturning moments. Tripods have the minimum number of piles and thus, each pile must carry more lead than would a structure having a base with more than three legs, assuming both the tripod and the structure with more legs were both evenly loaded.
The installation cost of offshore platform jackets is a major factor. A tripod jacket sometimes must be fabricated in a position in which its vertical axis is rotated 70 to 90 degrees toward horizontal, and is transported to the final erection site in that orientation. Tripods are lifted from the transportation vessel and placed in the body of water. Tripods are unstable in water when floating in the rotated position. They tend to roll to one side, which presents safety hazards and causes extra time and costs in rigging the lifting slings, work platforms and other apparatus. Jackets are temporarily supported by mudmats which rest on the ground below the body of water. Mudmats are most effective when placed at the outer corners of the base. The mudmats of a structure having more than three legs are more efficient than the mudmats of a similar sized tripod for the same reasons that apply to efficiency of piles. The combination of a tripod (also called a jacket structure) and base that is more efficient in distributing applied load; more stable when floating; and is more stable when resting on mudmats would be highly desirable.
Pile efficiency and jacket efficiency are significantly affected by the geometrical arrangement of piles. Current technology for tripods has piles near or within the jacket legs. Current technology does not use piles or pile groups as effectively as possible by allowing one to transition from a base configuration to a jacket configuration that is different from the base configuration. Consider a triangular base with each side having a length, L, and a rectangular base with each side having a length, L. There would be a pile or pile group at each corner of each base. If each base were to resist the same overturning moment, then the maximum reaction for the triangular base would be 1.4 times the maximum reaction for the rectangular base. The pile foundation must carry vertical loads in addition to overturning moment. Each pile of the triangular base would carry 1.33 times the load for the rectangular base, assuming the vertical load passed through the combined centroid of the piles of the base. The costs for fabrication, handling, and installation of piles would be significantly higher for the triangular base than for the rectangular base. The principles apply to tripods with other types of polygonal bases.
The prior art does not disclose improving the efficiency of load distribution to the structure and piles by arranging braces on the faces of the jacket so that well conductors may start outside the face of the jacket above the water line and then pass through the face of the jacket to be inside the perimeter of the jacket the remainder of the way to the ground below the body of water. A jacket design that allows the well conductors to pass through the jacket and terminate outside the jacket above the water""s surface would be highly desirable.
It is an object of the present invention to provide a jacket structure having three legs combined with a base which resists applied loads more efficiently.
It is another object of the present invention to provide improved efficiency of a support structure and/or piles by configuring the base of the structure such that the base has a larger number of legs than the structure above the base has.
It is a further object of the present invention to provide a structure that permits the well conductors to start outside the face of the jacket above the water line and then pass through the face of the jacket to be inside the perimeter of the jacket.
A support structure for use in drilling and production operations having one end positioned above a body of water and another end below the body of water on a bed is disclosed. Examples of such structures are deck platforms for supporting drilling rigs and production facilities. The support structure includes a base and a jacket structure having at least three jacket legs and a support means for supporting a structure such as a deck. The base has at least four cylindrical base legs engageably positioned in a generally rectangular pattern on the bed and a base frame connected to the cylindrical base legs. The cylindrical base legs start and terminate below the body of water. Preferably, each of the cylindrical base legs is sized to engageably receive at least one pile, and the pile is adapted to be driven into the bed.
The jacket has a first jacket leg, a second jacket leg, a third jacket leg. The first, second, and third jacket legs start below the body of water and terminate above the body of water and are positioned with the support means for supporting a structure above the body of water. The first jacket leg may be attached to one of the at least four cylindrical base legs and the second jacket leg may be attached to another of the at least four cylindrical base legs. The base frame extends between the cylindrical base legs to which the first jacket leg and the second jacket leg may be attached to the third jacket leg to form a triangular shape. The triangular shape, preferably, is a substantially isosceles triangle.
The support structure described herein has a base and jacket structure that is supported by piles, also referred to as skirt piles. Skirt pile sleeves are an integral part of the legs of the base of the structure. To secure the base to the bed, skirt piles are driven through the skirt pile sleeves. Each skirt pile is securely connected to a skirt pile sleeve. Methods of connection between pile and sleeve are mechanical, grouted or welded. Each pile is driven into the ground below the body of the water to a distance of penetration calculated sufficient to safely carry the applied loads.
The invention provides improved efficiency of the structure and/or piles by configuring the base of the structure such that the base has a larger number of legs than the structure above the base has, and the legs are placed at the outer corners of the base perimeter. The base has at least four legs, at least four sides, and at least four corners. The jacket has three legs. The invention improves the efficiency of the structure and/or piles by also providing a means for conductors to start outside a jacket, pass through the face of the jacket, and be within the perimeter of the jacket at the ground below the body of water The invention has an additional advantage of having more stability than a rotated floating tripod, making installation safer and cheaper than for a standard tripod. The invention is also more stable than a tripod of the same size when resting on mudmats. No extraordinary means or methods of fabrication or installation are required to build the invention and thus no extra costs are incurred.